The present invention generally relates to an electrochemical cell and, more particularly, relates to an improved electrode structure for an electrochemical cell.
Alkaline electrochemical cells (i.e., batteries) generally include a positive electrode, commonly referred to as the cathode, and a negative electrode, commonly referred to as the anode, arranged in a steel can and separated by a separator. The anode, cathode, and separator simultaneously contact an alkaline electrolyte solution which typically includes potassium hydroxide (KOH). In many conventional alkaline electrochemical cells, the cathode typically comprises manganese dioxide (MnO2) as the electrochemically active material, and further includes graphite and other additives. The anode typically comprises zinc powder as the electrochemically active material. In addition, a gelling agent is also typically included in the anode to suspend the zinc powder in a gelled electrolyte mixture. The separator is disposed within the inside of the positive electrode to physically separate the positive electrode from the negative electrode while allowing ionic transport between the two electrodes.
The negative electrode is typically formed by mixing the zinc active material in the form of a zinc alloy powder with the alkaline electrolyte in a gelling agent. The zinc powder mix is dispensed within the hollow central volume defined by the interior surface of the separator within the positive electrode. Subsequently, a collector assembly is inserted into the open end of the steel can with a current collector nail extending down within the negative electrode/electrolyte gel. An outer cover is then placed over the collector assembly and the can walls are crimped over the outer cover to seal the cell can closed. It is generally known that, in the manufacture and use of electrochemical cells employing zinc particles, the lowest zinc volume percent in the negative electrode that manufacturers typically utilize is about no less than twenty-eight percent (28%) in the negative electrode gel in order to both match the rate of electrochemical output of the positive electrode and provide sufficient particle-to-particle and particle-to-collector contact to maintain the electrical conductance of the negative electrode. Below this amount of zinc particles, voltage instability typically occurs, as well as a resulting production of a cell structure having high sensitivity to shock and vibration, which cause the zinc particles to migrate away from the current collector and to lose particle-to-particle contact, thereby decreasing cell efficiency.
In order to provide the maximum electrochemical activity and a minimum of limiting polarization, it is desirable to operate a battery at as low a current density on the zinc as possible while still producing the required amount of total current from the system. Accordingly, alkaline batteries conventionally employ electrodes made from powdered active materials to obtain the highest possible surface area per unit weight or volume, and thus minimize the current density. Conventional zinc powder is powder that has been produced by air-jet atomization of molten zinc, thereby providing irregularly shaped particles. While zinc powder negative electrodes are relatively efficient at low discharge rates, such electrodes are much less efficient when discharged at high rates. Given that most new battery powered devices generally have high current demands, causing the batteries to discharge at high rates, there exists a strong demand for batteries having greater high-rate performance.
In addition to zinc powder, it is also generally known to employ zinc flakes having a thickness many times smaller than the length and width. However, while the use of zinc flakes improves the high-rate performance of the negative electrode of an alkaline electrochemical cell, there remains room for further improving negative electrode performance, particularly at high drain rates.
It has been discovered that the discharge of zinc in an alkaline cell generally starts near the positive electrode and then proceeds away from the positive electrode. Because the reaction product (e.g., zinc oxide and zinc hydroxide) resulting from the discharge of zinc is more voluminous than the zinc itself, a reaction product skin tends to form between the positive and negative electrodes if there is not enough space to accommodate the reaction product. While such a skin still allows some electrolyte to pass through, the reacting zinc behind the skin does not receive hydroxyl ions from where they are formed in the positive electrode fast enough to offset those consumed by the reacting zinc. As a result, polarization occurs, leading to premature cell failure.
In many cell designs, the current collector, which is often in the form of a nail, is located in the center of the negative electrode. Because most of the zinc discharge occurs at the outer periphery of the negative electrode near the positive electrode interface, it is necessary to maintain a continuous path of connected zinc from the reacting site to the collector nail so as to facilitate electron transfer. When zinc powders or flakes are used, many particles must touch to form an electron conduction path back to the collector nail. However, because the zinc powder or flakes only constitute approximately thirty percent (30%) of the negative electrode volume, any physical shock to the cell may cause the particles to shift and lose contact. Thus, excess zinc is often added to the negative electrode only to serve as an electron conductor. The excess zinc, however, is not discharged during the life of the cell and takes up valuable space within the cell that could otherwise be used for extra electrolyte to fuel reactions or to hold discharge reaction product while still leaving space for ion transfer. Alternatively, some of the space could be used to increase the amount of active material (e.g., MnO2) in the positive electrode.
U.S. Pat. No. 6,150,052, entitled xe2x80x9cELECTRODE FOR AN ELECTROCHEMICAL CELL INCLUDING STACKED DISKS,xe2x80x9d by Lewis F. Urry, teaches a negative electrode formed with a plurality of individually stacked zinc disks. The aforementioned patent disclosure is hereby incorporated by reference. The stacked zinc disks generally discharge with enhanced efficiency at high currents, as compared to a homogenous suspension of zinc powder. While enhanced performance is achieved with the zinc disks, it is desirable to provide an electrode structure that is easy to manufacture and assemble in an electrochemical cell, and which achieves high efficiency at high discharge rates.
The present invention improves the performance of an alkaline electrochemical cell for at least high rate service and provides an electrode that is relatively easy to assemble. To achieve these and other advantages, the present invention provides for an electrochemical cell having a container, a first electrode disposed in the container, a second electrode disposed in the container, a separator located between the first and second electrodes, and an electrolyte. The first electrode has a wall defining an interface surface. The second electrode includes a unitary piece of electrochemically active material having a circumferential surface with multiple openings formed in the circumferential surface.
According to a first aspect of the present invention, the unitary piece of electrochemically active material comprises a slotted tube having a spine and a plurality of ribs supported by the spine. According to another aspect of the present invention, the unitary piece of electrochemically active material comprises first and second members each having a spine and a plurality of ribs. According to a third aspect of the present invention, the unitary piece of electrochemically active material comprises first and second members each comprising a folded sheet of electrochemically active material. According to a fourth aspect of the present invention, the unitary piece of electrochemically active material comprises a coiled piece of material having overlapping layers.